Field of the Invention and Related Art Statement
The present invention relates to a method of writing data on an optical card.
There have been proposed various types of data writing and reading apparatuses using optical cards of write-once-type. FIG. 1 is a block diagram showing schematically an example of the known writing and reading apparatus. The apparatus comprises an optical unit 1 which includes a semiconductor laser for emitting a data writing laser beam, a light emitting diode for emitting a data reading light beam, a light receiving element for receiving the data reading light beam modulated by data recorded on the optical card, an optical system for projecting the writing or reading beam onto the optical card and guiding the reading light beam reflected by the optical card onto the light receiving element, means for effecting focusing and tracking of the beams with respect to tracks on the optical card, and means for driving the optical system and optical card relative to each other. The optical unit 1 is well known in the relevant field of art and has been described in many publications, so that its detailed construction and operation may be dispensed with.
Now, the operation of the apparatus shown in FIG. 1 will be explained also with reference to a flow chart illustrated in FIG. 2.
Upon reading the data written on the optical card, an output signal supplied from the light receiving element in the optical unit 1 is amplified by a pre-amplifier 2 and then is converted into a bivalent signal in a signal processing circuit 3. The bivalent signal thus reproduced is supplied to a demodulation circuit 4 to demodulate the modulated data. The data thus demodulated is stored in a sector buffer memory 5B.
When writing the data on the optical card, the so-called read-after-write mode is adopted. Data to be written on the optical card is read out of a sector buffer memory 5A and is modulated in a modulation circuit 6. The modulation may be carried out by various methods. For instance, the data signal may be modulated in accordance with MFM (Modified Frequency Modulation). Then, the modulated data is supplied to a semiconductor laser driving circuit 7 to control an output current supplied from the driving circuit to the semiconductor laser in such a manner that the output power of the semiconductor laser provided in the optical unit 1 is changed to a writing level which is higher than a threshold level at which a record layer of the optical card becomes sensitive to light. In this manner, the data is written on the optical card (ST1). Immediately after the writing, the data which has been just recorded is read out of the optical card (ST2). The read out data is processed by the pre-amplifier 2, signal processing circuit 3 and demodulation circuit 4, and the demodulated data is stored in the sector buffer memory 5B. Then, the data stored in the sector buffer memory 5B is compared with the original data stored in the sector buffer memory 5A with the aid of the control circuit 8 to confirm whether the data has been written on the optical card correctly or not (ST3). When it is judged that the data has not been correctly recorded, the relevant track number is stored in the host computer (ST4), and after increasing the track number by one (ST6) to index a next track with respect to the optical unit, the same data is recorded again on the optical card in the next track. In this manner, the data writing operation is retried several times, while the output power of the semiconductor laser remains the same as that in the previous data writing operation. Then, the data thus recorded is reproduced to confirm the writing condition again. When the correct writing is not confirmed after a predetermined number of retries (ST5), the control circuit 8 sends to the host computer message announcing that the data writing has been failed (ST7).
The data writing method based on the above mentioned read-after-write mode has been disclosed in, for instance, Japanese Patent Publication Kokai 61-208,671.
Usually the optical cards are carried by users and the temperatures of the optical cards, particularly record layers of optical cards differ from each other. Further, the sensitivity of the record layers of optical cards might be different for respective cards. Therefore, even if the writing operation is repeated a predetermined number of times, while the output power of the semiconductor laser remains constant, the writing may not be performed correctly. This results in that the tracks on the optical card may not be utilized efficiently. In general, the record layer of an optical card is formed by a thin metal film and the laser beam is used to melt the metal film selectively in accordance with the data to be recorded to form openings in the metal film. The openings are generally called bits or pits and have optical properties different from that of the metal layer. Usually, the output power of the laser beam is set to such a value that the metal film at room temperature can be melted to form a pit having a predetermined dimension. When the temperature of the optical card is low, there might be formed in the metal film pits having smaller dimension than the predetermined dimension, and the pits might have a shorter length than the nominal value. On the contrary, when the temperature of the record layer of the optical card is high, the output power of the writing laser beam becomes too high to form pits or dots having the given dimension and the pits usually become longer than the nominal size. Usually, the data is written on the optical card in such a manner that the length of pits and/or the length of vacant spaces between successive pits is modulated in accordance with the modulated laser beam. Therefore, when the length of the pits is changed to extend beyond a given range of the length, it is no longer possible to recover the original data.